There is currently great interest in titanium aluminide (Ti aluminide) compounds possessed of low density and high strength relative to other available alloys. This interest is due to the continuous creation of new applications for these alloys, for example in the aerospace industry. These new applications generate an ever-increasing need for low density structural materials of high temperature strength, tolerable low temperature ductility, and reasonable oxidation resistance. This need of the industries drives ongoing research attempts to improve the titanium aluminide compounds. Since the microstructure of an intermetallic alloy affects its physical properties, these improvement attempts have focused on altering the microstructure of titanium aluminide compounds. The intermetallic phases .alpha..sub.2 -Ti.sub.3 Al (DO.sub.19) and .gamma.-TiAl (Ll.sub.O) have been extensively exploited for this purpose.
There are primarily two approaches that have been used in the development of improved titanium aluminide compounds. In the first approach, binary micro-structures composed of DO.sub.19 and Ll.sub.0 phases have been altered by adding small quantities of elements which modify phase boundaries and properties (U.S. Pat. No. 4,983,357; Proceedings of International Symposium on Intermetallic Compounds -Structure and Mechanical Properties, ed. O. Izumi, The Japan Institute of Metals (1991)).
In the second approach, niobium (Nb) has been added to increase plasticity of the DO.sub.19 phase, and to form microstructures combining the DO.sub.19 phase with .beta.(BCC)/B2 phases in the Ti-Al-Nb system (U.S. Pat. No. 4,292,077; R.G. Rowe in High Temperature Aluminides and Intermetallics, (C.T. Liu et al., eds) TMS-AIME, Warrendale OH, 375 (1990); Proceedings of International Symposium on Intermetallic Compounds--Structure and Mechanical Properties, ed. 0. Izumi, The Japan Institute of Metals (1991)). Using this latter approach, alloys approaching a composition consisting in atomic percent of 24% aluminum, 11% niobium, and balance titanium were found to possess very promising combinations of specific strength and rupture life at temperatures less than 800.degree. C.
Within the Ti-Al--Nb system, an ordered ternary Ti.sub.2 AlNb (0) phase was recently discovered that may have potential as a structural material for use at elevated temperature (D. Banerjee et al. Acta. Metal., 36, 871 (1988)). This O phase has orthorhombic symmetry (Cmcm) which occurs by Ti/Nb ordering of the hexagonal DO.sub.19 phase, or by Al/Nb ordering of the cubic B2 phase. Alloys with microstructures consisting of the O and .beta. (BCC)/B2 phases have been shown to possess excellent combinations of room and high temperature mechanical properties (U.S. Pat. No. 5,032,357). These alloys consist in atomic percent of 18-30% Al, 18-34% Nb, and balance Ti.
Another ternary phase in the Ti-Al-Nb system was also recently discovered. This phase is close in composition to Ti.sub.4 Al.sub.3 Nb. The ternary phase is in apparent equilibrium with the Ll.sub.0, DO.sub.19 and orthorhombic O phases (Bendersky et al., Acta Metall., 38, 931 (1990); Bendersky et al., Mat. Sci. Eng., A152, 41 (1992)). The phase has the B8.sub.2 structure of the omega-type phases (Bendersky et al., Acta Metall., 38, 931 (1990)). It forms readily at high temperature from the cubic B2 phase by displacement of pairs of 111 planes and subsequent chemical ordering.
Alloys comprised of a combination of this recently discovered omega-type B8.sub.2 phase and orthorhombic 0 phase are the subject of U.S. Pat. No. 5,190,602 (Bendersky et al.). These alloys exhibit physical characteristics of both the orthorhombic and omega-type phases and possess superior high-temperature strength and stability, and low density. The alloys consist in atomic percent of about 48-52% Ti, about 28-32% Al, and about 16-20% Nb.
Despite the development of these improved alloys, there remains a need for yet more improved alloys. Consequently, it is an object of the present invention to provide a novel Ti aluminide alloy with improved properties. It is a further object of the present invention to provide a Ti aluminide alloy with improved high and room temperature mechanical properties. It is another object of the present invention to provide a high-strength Ti aluminide alloy of lower density and improved corrosion resistance.
The present invention provides for such a Ti aluminide alloy which has improved high and room temperature mechanical properties, low density, and improved corrosion resistance. These and other objects and advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.